1. Field of the Invention
This invention relates new aqueous polyurethane dispersion adhesive compositions which can be an one component or a two component for use in thermoforming. More particularly, this invention relates to aqueous polyurethane dispersion adhesive compositions comprising carboxylic acid and/or its salt-containing sulfonated polyurethane dispersions, and with or without compatible crosslinking agents. These new adhesive compositions exhibit good stability at low pH values, long pot life, high green strength, good water resistance and improved heat resistance.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes are known and are described in, for example, U.S. Pat. No. 3,479,310; Angew. Chem., 82, 53, (1972); and Angew. Makromol. Chem., 98, (1981).
U.S. Pat. No. 3,479,310 to Dieterich et al discloses dispersing in water a fully chain extended, NCO-free polyurethane having internal ionic salt groups.
For anionic aqueous polyurethane dispersions, the ionic salt groups are salts of carboxylic acid or sulfonic acid.
Aqueous polyurethanes having only internal carboxylate groups as anionic moieties are stable only at pH&gt;7. Such polyurethane dispersions can form a two component adhesive composition with a compatible crosslinker, such as polyfunctional isocyanates, epoxies or aziridines.
Henning, et al., U.S. Pat. No. 4,870,129, discloses use of the sodium salt of N-(2-aminoethyl)-2-aminoethane sulfonate acid (AAS salt) to prepare polyurethane dispersions. The aqueous polyurethane dispersions are reported to have exhibited good stability at low pH values (5-7), high green strength and medium heat resistance.
Usually, after blending the sulfonated polyurethane dispersions with a polyisocyanate crosslinker, such as the dispersible polyisocyanate in U.S. Pat. No. 4,663,377 to Hombach et al, their heat resistance will be improved.
Leung, U.S. Pat. No. 4,762,880, discloses water-based thermoforming adhesives comprising aromatic polyurethanes, cross-linking agents and others. These kind of adhesive compositions will need high temperature to activate due to the aromatic polyurethane component.
U.S. Pat. No. 4,870,129 to Henning et al discloses an adhesive consisting of an aqueous polyurethane containing chemically incorporated carboxylate or sulfonate groups. The adhesive of this reference showed low activation temperature but only medium heat resistance.
Duan et al in copending application Ser. No. 08/126,508, filed Sep. 24, 1993, disclose aqueous polyurethane dispersions based on sulfonated polyester polyols, which have an unusually high crystallization rate, while also exhibiting good stability at low pH values, high green strength and medium heat resistance.